Digital subscriber line (DSL) technologies can provide a large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent twisted-pair phone lines, for example in a same or nearby bundle of lines. Crosstalk, including near-end crosstalk (NEXT) and far-end crosstalk (FEXT), may limit the performance of various DSL systems such as those defined by standards including asymmetric DSL 2 (ADSL2), very high speed DSL 2 (VDSL2), and G.fast (future standard). In use, crosstalk can be reduced or canceled by joint processing of signals in multiple subscriber lines. Depending on whether the signals are in a downstream or upstream direction, a crosstalk precoder or canceller may be used on an operator's end of a DSL system, such as a digital subscriber line access multiplexer (DSLAM). For example, crosstalk precoding is a technique in which downstream signals are pre-distorted prior to transmission through a binder. A pre-distortion filter or ‘precoding matrix’ is used to pre-distort the signals, and thus cancel FEXT that occurs between subscriber lines in the binder. The signals may then arrive at receivers located at different customer sites with less or no FEXT, thereby achieving higher data-rates.
Broadband access communication technologies, such as very-high-speed digital subscriber line (VDSL), VDSL2, and future standard G.fast to be issued by International Telecommunication Union Telecommunication Standardization Sector (ITU-T) Study Group 15 (SG15), may provide data for triple-play services. For example, television, internet, voice over internet protocol (VoIP) phone services may all be supported. Channel capacity in the physical media dependent (PMD) layer of a DSL system may be high (e.g., near gigabits in G.fast) in the case of a single subscriber line. However, when multiple subscriber lines are deployed in a same binder, actual data rate may be lower than the channel capacity due to NEXT and/or FEXT.
In a DSL system, NEXT may be reduced or canceled via the use of synchronous time division duplexing (STDD). In the STDD mode, all subscriber lines connected to, for example, a transceiver (transmitter and receiver) located in a customer premise equipment (CPE) may be configured to either transmit upstream signals or receive downstream signals at any given time, but not simultaneously. A transceiver located in a DSLAM may be configured similarly. Therefore, for the transceiver either in a DSLAM or CPE, it may either be in a transmitting mode or receiving mode. Downstream and upstream time division may allow a transceiver to avoid its own transmitter echo, and STDD may help prevent NEXT between subscriber lines.
In use, FEXT mitigation may require estimation of downstream and upstream FEXT channels. For instance, in the ITU-T G.993.5 Recommendation, a FEXT training signal may be sent during a sync symbol (SS). A plurality of sync symbols corresponding to a plurality of subscriber lines may be modulated by an orthogonal sequence. The length of the orthogonal sequence may be proportional to a number of subscriber lines. As the number of subscriber lines increases, a training time may increase, and a total level of FEXT in the subscriber lines may also increase. Sometimes, the FEXT level may become stronger than a received data signal. In this case, the training of a frequency domain equalizer (FEQ) and a FEXT precoder/canceller may not work well, which results in loss of system performance. Further, FEXT may be relatively stronger in high frequency subcarriers. Thus, the training problem or issue may be worse in DSL systems, e.g., G.fast, which may increase the high frequency band edge from, for example, 17/30 megahertz (MHz) used in VDSL2 to 100 MHz or higher. Thus, there may be a need for improved FEXT compensation in DSL systems.